1. Field of the Invention
The present invention relates to a vane type vacuum pump to be mounted on diesel engine automobiles and the like, and more specifically, to a vane type vacuum pump which enhances a volume efficiency as well as realizes miniaturization and reduction of weight by improving the coupling structure of an input shaft with a rotor.
2. Description of the Related Art
In general, although automobile brakes use a negative pressure to obtain an auxiliary force, since diesel engine automobiles cannot obtain the negative pressure directly from an engine, they obtain the negative pressure by driving a vacuum pump by the rotational torque of the engine.
FIG. 10 is a side cross sectional view showing a vane type vacuum pump employing a conventional gear drive system and FIG. 11 is a cross sectional view taken along the line D--D of FIG. 10.
In the respective drawings, the input shaft 1 of the vane type vacuum pump has a gear la securely fixed to the outer periphery of one end thereof projecting to the outside by force-fit (or shrinkage-fit) and the vacuum pump is driven in rotation by an engine side drive gear (not shown) engaged with the gear 1a.
The end of the input shaft 1 on the gear la side is journaled on a bearing 2a disposed to a front bracket 2 and the other end of the input shaft 1 is journaled on a sleeve bearing 3a disposed to a rear bracket 3. The front bracket 2 and the rear bracket 3 are composed of aluminum alloy for the reduction of weight thereof.
The front bracket 2 and the rear bracket 3 constitute a cylindrical pump chamber P which is hermetically sealed through an O ring 4 and the central axis of the pump chamber P is offset from the center of rotation of the input shaft 1.
In this case, the rear bracket 3 has a housing shape for constituting the pump chamber P and is securely fixed to the front bracket 2 by three bolts Q (see FIG. 11.)
A spline-coupling portion 1b is formed to the middle of the input shaft 1 in the pump chamber P and a rotor 5 is engaged with the input shaft 1 through the spline-coupling portion 1b. A similar spline-coupling portion is also formed to the inner periphery of the rotor 5 so that it is coupled with the spline-coupling portion 1b.
The rotor 5 is held unmovable in a rotational direction and movable in an axial direction by being spline coupled to the input shaft 1.
The input shaft 1 and the rotor 5 are composed of alloy steel or a sintered iron (Fe) material to secure strength at the spline-coupling portion.
A plurality of grooves 5a (three in the case of FIG. 11) are formed to the outer periphery of the rotor 5 and vanes 6 are accommodated in the respective grooves 5a so as to be radially movable.
As shown in FIG. 11, one end of the rotor 5 comes into intimate contact with the inner wall of the rear bracket 3 through an oil film of several microns thick and the vanes 6 are completely accommodated in the grooves 5a at the position. Further, a space is formed by offset between each the grooves 5a and the inner wall of the rear bracket 3 so that the vane 6 can sufficiently fly out from the groove 5a.
A lubrication passage 3b and a suction port 3c are formed to the rear bracket 3 and an exhaust port 2b communicating with the lubrication passage 3b is formed to the front bracket 2. A lubricant supplied from the lubrication passage 3b acts to seal the pump chamber P as well as cool the vacuum pump and is exhausted from the exhaust port 2b passing through the sleeve bearing 3a and the spline-coupling portion 1b.
A tank (not shown) which is required to be evacuated to vacuum is connected to the suction port 3c and air sucked from the suction port 3c (see an arrow) is exhausted from the exhaust port 2b disposed at a confronting position.
Next, operation of the conventional vane type vacuum pump shown in FIG. 10 and FIG. 11 will be described.
First, the input shaft 1 journaled on the bearing 2a and the sleeve bearing 3a is driven in rotation through the gear la engaged with the engine side drive gear.
The rotational torque of the input shaft 1 is transmitted to the rotor 5 through the spline-coupling portion 1b and rotates the vanes 6 disposed in the outer peripheral grooves 5a of the rotor 5 clockwise in FIG. 11.
With this operation, the vanes 6 are flown out radially from the grooves 5a by a centrifugal force and rotated while being pressed against the inner wall of the rear bracket 3 of the pump chamber P under pressure as well as sliding through the oil film.
At the time, since the center of rotation of the rotor 5 is offset from the central axis of the pump chamber P, air is sucked from the suction port 3c side and sequentially exhausted from the exhaust port 2b side as the vanes 6 are rotated to thereby make the tank connected to the suction port 3c to a negative pressure.
Incidentally, the volume efficiency of the vacuum pump depends upon the amount of offset of the center of rotation the rotor 5 with respect to the pump chamber P (the amount of flying out of the vanes 6), that is, the depth of the outer peripheral grooves 5a of the rotor 5. However, since the depth of the grooves 5a is regulated by the outside diameter of the spline-coupling portion 1b and further the outside diameter of the spline-coupling portion 1b cannot be reduced to secure mechanical strength, it is difficult to set the volume efficiency depending upon the pump chamber P and the vanes 6 to a large value.
Since the rotor 5 which is engaged through the spline-coupling portion 1b in the vicinity of the center of the input shaft 1 is composed of the sintered Fe material to secure the mechanical strength such as wear resistance and the like taking transmission torque into consideration, it is difficult to reduce the weight thereof.
Further, since the coefficient of thermal expansion of the rotor 5 composed of the sintered Fe material is different from that of the front bracket 2 and the rear bracket 3 composed of the aluminum alloy, it is difficult to secure gastightness in the pump chamber P of high temperature, thus sufficient vacuum characteristics cannot be obtained at high temperature.
As described above, the conventional vane type vacuum pump transmits the rotational torque of the input shaft 1 to the rotor 5 by the engagement of the spline-coupling portion 1b formed to the outer periphery of the input shaft 1 with the spline-coupling portion formed to the inner periphery of the rotor 5. Thus, there is a problem that the volume efficiency cannot be sufficiently secured by the vanes 6 in the pump chamber P because the depth of the grooves 5a are regulated.
Since the rotor 5 which is rotated through the spline-coupling portion 1b is composed of the sintered Fe material, there is a problem that the weight of the rotor 5 cannot be reduced.
Further, since the coefficient of thermal expansion of the rotor 5 (sintered Fe material) is different from that of the front bracket 2 and the rear bracket 3 (aluminum alloy), there is a problem that it is very difficult to secure gastightness between the rotor 5 and the pump chamber P in a temperature range to be used.
An object of the present invention made to solve the above problems is to provide a vane type vacuum pump which enhances a volume efficiency as well as realizes miniaturization and reduction of weight.
Another object of the present invention is to provide a vane type vacuum pump which realizes reduction of weight as well as secures stable gastightness in a wide temperature range by composing a rotor of aluminum alloy.